1. Technical Field
Aspects of the present invention relate to a facsimile device used in a situation in which the facsimile device is connected to a public line in parallel with a telephone, particularly to a facsimile device having a printing function and a scanning function.
2. Description of Related Art
A facsimile device is configured to communicate with an external device through the public line. Therefore, in general, a facsimile device is used in an office or a home in a situation where the facsimile device is connected to the public line in parallel with a telephone. From the viewpoint of such a use environment of facsimile devices, a facsimile device having the function of judging whether a telephone (i.e., whether the telephone is in an on-book state) closes the public line has been proposed.
An example of such a facsimile device is disclosed in Japanese Patent Provisional Publication No. 2004-207809 (hereafter, referred to as JP 2004-207809A). According to JP 2004-207809A, the facsimile device is configured to detect a voltage of the public line to which the facsimile device and a telephone (an external telephone) are connected. The facsimile device judges that the external telephone closes the public line (i.e., the external telephone is in an on-hook state) if the facsimile device releases the public line and the voltage of the public line is lower than or equal to a predetermined voltage, while the facsimile device judges that the external telephone releases the public line (i.e., the external telephone is in an off-hook state) if the facsimile device releases the public line and the voltage of the public line is higher than the predetermined voltage.
Meanwhile, there is a case where the facsimile device is not able to appropriately judge whether the external telephone releases the public line if a digital service unit such as a PBX (Private Branch Exchange) or a terminal adapter intervenes between the public line and communication devices (i.e., the facsimile device and the external telephone). FIG. 5 is a graph illustrating change of the voltage of the public line in a situation where a digital service unit does not intervene between the public line and the communication devices. As shown in FIG. 5, if the facsimile device or the external telephone transfer to a state where the facsimile device and the external telephone are capable of conducting data communications, the public line is closed and the voltage of the public line decreases from a standby voltage Vr to a certain low voltage. If the public line is released, the voltage of the public line recovers to the standby voltage Vr.
In this situation, the facsimile device is able to appropriately judge that the external telephone closes the public line (i.e., the external telephone is in an on-hook state) if the voltage of the public line is lower than a predetermined threshold Vt, while the facsimile device is appropriately judge that the external telephone releases the public line (i.e., the external telephone is in an off-hook state) if the voltage of the public line is higher than the predetermined threshold Vt.
On the other hand, if a digital service unit intervenes between the public line and the communication devices, the voltage of the public line may change as shown in FIG. 6. That is, FIG. 6 is a graph illustrating the case where the voltage of the public line does not recover quickly to the standby voltage Vr after the facsimile device releases the public line. In this case, the voltage of the public line does not recover to the standby voltage Vr until a predetermined time period has elapsed. That is, the voltage of the public line stays at an intermediate voltage, which is lower than the predetermined threshold Vt, until the predetermined time period has elapsed.
In this case, the facsimile device erroneously judges that the external telephone closes the public line even if the external telephone releases the public line because as described above the voltage of the public line is kept at the intermediate voltage lower than the predetermined threshold Vr in the predetermined time period. In this case, the facsimile device may erroneously display a message indicating that the external telephone is in a busy state despite the fact that the external telephone has already releases the public line.